1. Field of the Invention
The methane-containing, sedimentary formations are drilled to great depths, into or in search of gas caps which will deliver natural gas (primarily methane) to the surface. These wells are capped off and abandoned when the gas cap is exhausted or if no gas cap is located. With deep wells, e.g., at depths greater than 10,000 feet, this practice leaves unrecovered what is usually the major fraction of the natural gas penetrated. Specifically, it leaves behind (a) natural gas dissolved in the hot, high-pressured brine and (b) gas trapped as small bubbles in the pores of the host formation. If the dissolved gas is extracted in some way from its solution in the brine, then the entrapped bubbles will resupply the depleted gas dissolved in the brine, again creating a solution of natural gas in brine. In fact, sampling of the gas solutions in brine from actual wells confirms that the brine is indeed saturated with natural gas, consistent with the presence of both dissolved gas and bubbles in two thermodynamically different phases. Gas which is dissolved or entrapped in small pores is not considered recoverable on a commercial basis.
In explaining this invention it is useful to consider two representative sets of conditions downhole, one involving brine at normal hydrostatic pressures and the other dealing with brine at abnormal pressures because of some additional lithostatic pressure, i.e., geopressured. Under each set of conditions the natural gas is recoverable, but the environmental problems are greater for the geopressured case.
First, for the hydrostatically pressured case, assume the following conditions: the formation containing the brine has a porosity of 25% and a permeability of one darcy, the hydrostatic pressure is about 0.465 psia per foot of depth, so a well drilled to 15,200 feet has a bottom-hole pressure (BHP) of 7,065 psia. The temperature is 300.degree. F., and at this temperature the pressure of saturated steam over the brine is about 65 psia. In addition, the brine is saturated with natural gas so that its partial pressure of 7,000 psia plus that of the steam just equals the 7,065 psia BHP. The methane concentration to achieve saturation is 38 standard cubic feet (SCF) per bbl of brine at bottom-hole conditions. The brine is saturated with the solids making up the host formation, and additional solubility of CaCO.sub.3 and other carbonates results from the presence of dissolved CO.sub.2 which adds Ca(HCO.sub.3).sub.2 to the solution. Near-saturation quantities of CaSO.sub.4 and other sulfates may be present even though the solids do not exist in the formation. NaCl and other dissolved chlorides are not of great importance for the present analysis. Temperatures and pressures decrease at depths less than those at the bottom of the wellpipe, and the pressures of steam and methane fall to near zero at ground level or the ocean surface. In a theoretical sense these conditions are unstable because the hot brine could in principle move to a lower pressure region and discharge methane and steam. Here the hot brine and gases which could no longer be contained by the hydrostatic pressure head would rush up the wellpipe in a continuing action much like the action of a coffee percolator or geyser. In practice there is a vanishingly small likelihood that the upward flow of brine would initiate itself; if, however, the system is designed properly, and if the circulation is initiated, then this tendency to release methane can be used to circulate brines so that their dissolved methane can be removed in a special type of stripper. The potential to release steam must be suppressed because steam vaporization may lead to solid precipitation and plugging of the wellpipe. This invention describes a method to initiate and continue the circulation while suppressing the steam vaporization and wellpipe plugging.
The work used to circulate the brine is derived from two in situ sources, and these in situ sources of work can be supplemented from external sources such as engines at the earth or water surface. First, in situ, simultaneous and coupled release of virgin brine and reinjection of spent brine back into the formation are used to balance the release and injection forces, and, second, in situ, additional energy to overcome frictional forces is available from the release of methane and limited amounts of steam as pressure is reduced, and the expansion of these gases provides a fluid which can do useful work downhole. If the methane pressure is dropped from 7,000 psia to 14.7 psia (atmospheric pressure), then 38 SCF of natural gas per bbl of brine will be released. About 7.8% of the brine also will boil off before the temperature drops from 300.degree. F. to 212.degree. F. and atmospheric pressure is reached. This alteration of the brine will result in precipitation of solids both because the amount of water is decreased and because dissolved CO.sub.2 is removed by gas sweeping as steam escapes. If, however, the methane pressure is maintained high enough so that the total pressure is over 65 psia, then the 300-degree brine cannot boil and steam removal is greatly reduced. As a corollary little dissolved CO.sub.2 escapes and the brine concentrations are not altered so the solutions remain stable and solids do not precipitate. Furthermore, violent ejections of brine will be largely controlled if boiling is prevented. If, for example, the gas pressure is maintained at 100 psia, then over 99% of the dissolved methane can be released, and the gas released at 300.degree. F. from one bbl of brine will consist of 65 psia of steam plus 35 psia of methane jointly making up 100 psia of gas pressure in a total volume of 16 cu ft. In this case less than 0.01% of the brine boils away and no important precipitation of solids occurs. However, the volume of the fluid is essentially quadrupled (5.6 cu ft per bbl of brine to 21.6 cu ft for brine plus gas), and the gas volume at 100 psia is available to pump a third as much brine volume at 300 psia for circulation and injection of spent brine. In this case the brine can be withdrawn from a hot region, circulated through a methane stripper, and reinjected into a slightly different region. Because the formation is porous and the pressure is hydrostatic, brine will flow to equalize pressures rapidly, and there will be no subsidence.
Now consider a geopressured formation in which the pressure is partly hydrostatic at about 0.465 psia per ft of depth and partly lithostatic at about 1 psia per ft of depth. The well is 15,200 ft deep, the temperature is 300.degree. F. at the bottom of the hole, the BHP is 12,000 psia, and there is a methane solubility of 60 SCF per bbl. Release of this methane can produce 25 cu ft of methane plus steam at 100 psia. This gas volume at 100 psia can move the brine volume at about 450 psia. If the brines are stripped of their methane and then reinjected in to the same geopressured formation but at a remote region, so that removal and reinjection are hydraulically linked, then large regions of the formation can be made available for methane recovery while the chance of serious subsidence is much reduced. This opportunity for limiting subsidence in geopressured regions by reinjection of the spent brine back into its original formation is solved by this invention.
To prevent collapse of the wellpipes, it is necessary to design the system so that the high pressure differences between the formation pressure and the pressure of the product methane do not act against the wellpipe.
Because cooling the saturated brines can lead to precipitation of solids, the cooling should be minimized, thus the methane is extracted in the hot regions of the wellpipe rather than where ocean or ground water has cooled it.
2. Prior Art
A. "Method and Devices for In Situ Recovery of Gaseous Hydrocarbons and Steam," by G. R. B. Elliott, N. E. Vanderborough, and M. W. McDaniel, Patent application Ser. No. 15,360, filed Feb. 26, 1979. This patent application recognizes the value of recovering in situ energy to assist in the reinjection of spent brine after methane removal, and it points out the value of recovering the methane without ever bringing brine to the ocean or earth surface. It does not disclose that the release of dissolved methane can suppress solids precipitation from steam vaporization while supplying nearly all of the energy needed to circulate the brine.
B. "The Status of Dissolved Gas in Japan," by S. S. Marsden, in Petroleum Engineer, June 1980, pp. 23-34. This paper describes the Japanese production of methane from methane-containing brines which involves pumping brine to surface facilities where methane is stripped out. The pumps are not self-powered, and no deep, hot wells are involved; rather, the wells are typically at 1,500-3,000 feet depths with 6,000 feet being maximum depth.
C. U.S. Pat. No. 4,149,598 (4/79) and 4,149,596 by Christian et al. These patents show that methane can be recovered from aquifiers which are hydrostatically pressured and in this recovery, large quantities of brine are lifted to the surface by pumping from the surface. The pumping causes a pressure drop in the brine near the wellpipe and, so long as pumping is continued, dissolved methane can be released to a gas phase. If pumping is stopped, the pressure of the brine will again rise to the hydrostatic pressure head, and gas evolution will cease. The Christian patents do not recognize the value of using the expansion of the methane upon pressure release to drive the pumping necessary to circulate virgin brine into position for methane release, and the serious problem of disposing of the spent brine which is brought to the surface.
D. U.S. Pat. No. 3,782,468 by Kuwada. This work identifies that the evaporation of brine to steam can cause the circulation of hot brine to the surface for processing, and injection of CO.sub.2 is described to suppress the precipitation of carbonates and hydroxides which could form as steam release swept CO.sub.2 out of solution.
E. U.S. Pat. No. 4,131,161 by Lacquement. This patent describes the use of a standpipe inside a wellpipe to circulate brine and recover dry steam from deep, hot wells. This patent uses in situ forces to achieve the pumping to circulate brine, and all the steam recovery facilities are below ground. The work does not address serious limitations imposed on the invention if steam is released from hot, saturated brine, i.e., geyserlike ejection of brine up the wellpipe and precipitation of solids with wellpipe plugging.
F. "Petroleum Production Handbook," Editors T. C. Frick and R. W. Taylor, Chapter 6, "Hydraulic Pumps," by C. J. Coberly and F. B. Brown, and Chapter 31, "Wellbore Hydraulics," by J. K. Welchon, A. F. Bertuzzi, and F. H. Poettman. These chapters indicate the pumping and gaslift concepts which are used in oil and gas production.